The present invention relates to porous ceramics and a method of preparing the same, and more specifically, it relates to porous ceramics for serving as an electrical insulating material applied to various types of wiring circuit boards or a lightweight structural material having moisture absorption resistance.
The present invention also relates to a microstrip substrate employed for forming a waveguide of a high frequency of at least 1 GHz, particularly at least 30 GHz, and more specifically, it relates to a microstrip substrate having a microstrip line consisting of a conductor formed on the surface of a substrate consisting of porous ceramics.
Improvements of characteristics such as further weight saving and improvement in strength or improvements of electrical characteristics are recently required to ceramics employed as various structural materials or electronic component materials. In a wafer transfer stage or a plotting stage used as a component of semiconductor equipment, for example, further weight saving of a stage material is required for high precision and high-speed driving. As to a circuit board or an insulating material employed for an electronic apparatus, a material having lower permittivity and lower dielectric loss is strongly required following recent improvement in frequency.
Therefore, it is conceivably effective to use ceramics in a porous state. When the relative density of ceramics is reduced to 50%, for example, the weight thereof can be reduced to 50%. Further, the air exhibits excellent electrical insulation properties with permittivity of about 1 and dielectric loss of zero, and hence porous ceramics attains desirable characteristics as a material to which low permittivity and low dielectric loss are required.
However, it is difficult to obtain a porous sintered body containing homogeneously dispersed fine pores simply by controlling a sintering step for a ceramic sintered body. In general, coarse pores are formed to disadvantageously reduce strength or result in heterogeneous characteristics. Further, most of pores forming the obtained porous sintered body are open pores deteriorating humidity resistance intrinsic to the ceramics, and desired characteristics cannot be attained in view of practicality due to remarkable deterioration of electrical characteristics (permittivity and dielectric loss) resulting from moisture, dispersion of various characteristics or the like.
Therefore, various methods are devised for obtaining a porous material consisting of fine closed pores. For example, Japanese Patent Laying-Open No. 3-177372 discloses an SiC-based porous sintered body containing a phase having a different thermal expansion coefficient to exhibit a total volume ratio of closed pores of 0.07 to 27.5%, in order to improve toughness. When this method is employed for obtaining an SiC-based porous sintered body having closed pores in a ratio of at least 27.5%, however, oxidation resistance is reduced or the pore sizes are increased.
Japanese Patent Laying-Open No. 5-310469.discloses a high-purity calcia sintered body exhibiting a ratio of 5 to 15% as to closed pores of 2 to 10 xcexcm in diameter. This gazette states that this sintered body is obtained by mixing a foaming agent such as phenol aldehyde or flammable fine powder such as carbon black into muddy serosity of calcium carbonate and water and calcining the mixture. According to this method, however, residues of the foaming agent or the flammable fine powder remain in the closed pores while it is difficult to hold the shape if the amount of the foaming agent is increased, and hence the closed pore ratio cannot be increased.
Japanese Patent Laying-Open No. 6-157157 discloses lightweight and high-strength ceramics formed with closed pores prepared by equilibrating the pressure of the closed pores in the ceramics and the pressure in a calcinator. In this method, however, it is difficult to control the pore size.
According to Japanese Patent Laying-Open No. 11-116333, porous glass having closed pores of nanometer order is adjusted by phase-splitting borosilicate glass by heat treatment, eluting a soluble phase, performing pulverization and thereafter melting only the surface with a flame for forming closed pores. This gazette discloses a method of adjusting a mixture of glass, an aggregate and resin balls with a porous aggregate obtained by crystallizing/heat-treating the glass for preparing a ceramic circuit board by green sheet lamination. The ceramic circuit board obtained by this method exhibits relative permittivity of not more than 2 and a thermal expansion coefficient of 13 to 17 ppm/xc2x0 C. This method is restricted to a material phase-split by heat treatment for eluting a soluble phase. Further, not only the process is complicated but also composition with a different phase is necessary, and hence intrinsic mechanical and electrical characteristics cannot be obtained. When open pores are temporarily exposed to the atmosphere to cause adsorption of moisture or the like, further, it is difficult to completely dissociate and control this.
In the aforementioned prior art forming closed pores, a second phase, such as a foaming agent, a melt or a phase having a different thermal expansion coefficient, different from the matrix phase must be added and hence electrical and mechanical characteristics are remarkably reduced due to the second phase or residues of the second phase. Further, the ratio and the size of formable pores are limited such that no matrix skeleton can be formed or the pore size cannot be controlled if the porosity is increased.
Porous ceramics, having excellent characteristics such as a light weight, heat insulating properties, excellent workability, high dimensional accuracy (fow shrinkage factor) in calcination, low permittivity and the like, can be expected for application to various structural materials, members for a filter and a vacuum chuck and electronic materials such as an insulating member (substrate), a low dielectric loss member (substrate) and the like.
However, porous ceramics having an irregular surface resulting from pores is insufficient in surface accuracy, and inferior in abrasion resistance and frictional characteristics or adhesion, flatness, film density (pinholes), dimensional accuracy and humidity resistance in a case of forming a functional thin film or a conductive pattern on the surface in application to the aforementioned usage.
As a method of smoothing the porous surface, therefore, there has been reported a method of grinding/polishing the surface similarly to dense ceramics or a method of impregnating the surface of a porous material with a ceramic slurry and thereafter sintering the same for densifying the surface (Japanese Patent Laying-Open No. 61-53146,Japanese Patent Laying-Open No. 1-164783, Japanese Patent Laying-Open No. 1-215778 or Japanese Patent Publication No. 1-47435).
When merely working the material by a method similar to that for a dense body, however, it is difficult to obtain a smooth surface of sub-micron order due to remaining irregularity resulting from pores. When ceramics or glass grains are simply deposited on the surface of porous ceramics, it is difficult to obtain a sufficiently flat surface, and reliability is deteriorated due to falling of the deposited grains or the like.
When a dense ceramic sheet is stacked on the surface of a porous material or the porous material is impregnated with a dense ceramic slurry and calcined, stress is caused in a dense region of the ceramics and the porous region due to a large shrinkage factor of the dense ceramic part, and hence the matrix is deformed (warped), a target dense layer is not formed or readily separated, or sufficient smoothness cannot be attained. The number of steps in the fabrication process is increased, to result in inferior productivity or the like.
As a conventional high-frequency circuit board, a dielectric substrate is employed as a relay substrate for connecting a package and an integrated circuit (IC) with each other or a substrate for a hybrid IC formed by mounting an IC, resistors, capacitors and the like on a substrate, as shown in xe2x80x9cDetailed Explanation of Praxes-Exercises, of Microwave Circuitxe2x80x9d by Genzaburo Kuraishi, 1983, issued by. Tokyo Denki University Press or Japanese Patent Laying-Open No. 6-244298, for example. Alumina (Al2O3), glass, epoxy resin or the like is employed as the material for the dielectric substrate applied to such usage.
Among these materials, alumina is mostly employed as the material for the substrate as to application in a high frequency band of microwaves and millimeter waves. Alumina is employed for the following reasons:
(i) A resin material such as epoxy resin, exhibiting lower relative permittivity as compared with alumina, exhibits heat resistance of only about 250xc2x0 C. Therefore, the resin material cannot withstand a bonding temperature (about 320xc2x0 C.) for an Auxe2x80x94Sn alloy generally employed as a brazing filler metal for bonding a microwave IC.
(ii) When employing a substrate consisting or an organic material having a dielectric dissipation factor (tanxcex4) of 10 to 100 times that of a ceramic material, transmission loss is increased.
An attempt of employing various dielectric substrates is made in order to reduce a propagation delay time particularly in a mother board for a computer. Such a substrate material is prepared by mixing a material such as glass or resin having low relative permittivity into conventional ceramics (alumina).
However, the relative permittivity of glass is 4 to 5 or 3.5 at the minimum, and hence reduction of the relative permittivity of the substrate for reducing the propagation delay time is limited when glass is mixed into the substrate material. When a resin material is mixed into the substrate material, heat resistance of alumina employed as the main material is disadvantageously reduced.
In an example disclosed in Japanese Patent Laying-Open No. 3-93301 or Japanese Patent Laying-Open No. 5-182518, an organic material such as porous plastic or polymer resin is employed as the material for a dielectric substrate. Even if the relative permittivity of the substrate can be reduced for reducing transmission loss such as a signal transmission delay time by employing such a material, however, heat resistance for bonding an IC chip or the like cannot be attained.
While a dielectric substrate consisting of alumina is generally employed in a high frequency band of microwaves and millimeter waves, alumina exhibiting extremely high relative permittivity of about 9 to 10 has the following problems:
(a) An unwanted mode of electromagnetic waves is generated in a part of the circuit board coming into contact with the air having relative permittivity of 1 due to the remarkable difference in relative permittivity, to result in transmission loss.
(b) In the high frequency band of millimeter waves or the like, a dielectric waveguide further miniaturizable as compared with a waveguide is employed as the basic element of an integrated circuit. While a large number of types of dielectric waveguides are present, a microstrip line is employed as a dielectric waveguide having a basic planar structure suitable for integration.
When strip conductors are adjacently formed on a dielectric substrate in such a microstrip line, however, bonding capacity between the adjacent conductors is disadvantageously increased to readily cause mutual interference.
(c) In order to set the characteristic impedance to 50 xcexa9 in the microstrip line, the thickness of the dielectric substrate and the line width of the strip conductors must be set to 1:1. When employing a dielectric substrate having a small thickness, therefore, the line width of the strip conductors is inevitably reduced. Consequently, transmission loss is increased in the microstrip line while the precision of the line width exerts remarkable influence on fluctuation of the characteristic impedance.
According to the aforementioned xe2x80x9cDetailed Explanation of Praxes Exercises of Microwave Circuitxe2x80x9d, p. 187, the characteristic impedance Z0 can be calculated as follows:                               a          =                                    1              +                              1                /                                  ϵ                  r                                                      2                          ,                                            Δ              ⁢                              xe2x80x83                            ⁢              W                        t                    =                                    1              π                        [                          1              +                              ln                ⁢                                  4                                                                                                              (                                                      t                            h                                                    )                                                2                                            +                                                                        {                                                      1                                                          π                              ⁡                                                              (                                                                                                      W                                    t                                                                    +                                  1.1                                                                )                                                                                                              }                                                2                                                                                                                  ]                                                                        W            xe2x80x2                    =                      W            +                          a              ⁢                              xe2x80x83                            ⁢              Δ              ⁢                              xe2x80x83                            ⁢              W                                      ,                  b          =                                    (                                                14                  +                                      8                    /                                          ϵ                      r                                                                      11                            )                        ⁢                          (                                                4                  ⁢                  h                                                  W                  xe2x80x2                                            )                                                                        Z          0                =                              42.4                                                            ϵ                  r                                +                1                                              ⁢          ln          ⁢                      {                          1              +                                                (                                                            4                      ⁢                      h                                                              W                      xe2x80x2                                                        )                                ⁢                                  (                                      b                    +                                                                                            b                          2                                                +                                                  a                          ⁢                                                      xe2x80x83                                                    ⁢                                                      π                            2                                                                                                                                )                                                      }                              
where xcex5r represents the relative permittivity of the substrate, W represents the width of the line conductors (strip conductors), t represents the thickness of the line conductors and h represents the thickness of the dielectric substrate.
(d) According to p. 189 of the aforementioned literature, transmission loss in the microstrip line, more specifically the attenuation constant xcex1, is given as follows:   a  =                              72          ⁢                      xe2x80x83                    ⁢          K                          W          ⁢                      xe2x80x83                    ⁢                      Z            0                              ⁢                        f                      σ            T                                +          91      ⁢      f      ⁢                        ϵ          eff                    ⁢                        1          -                      (                          1              /                              ϵ                eff                                      )                                    1          -                      (                          1              /                              ϵ                r                                      )                              ⁢      tan      ⁢              xe2x80x83            ⁢              δ        ⁢                  xe2x80x83                [                  d          ⁢                      xe2x80x83                    ⁢          B          ⁢                      /                    ⁢          m                ]            
where xcex5eff represents the effective relative permittivity of the line, xcex5r represents the relative permittivity of the dielectric substrate, tanxcex4 represents the dielectric dissipation factor,"sgr"T represents the specific conductivity of the conductors (specific conductivity of the conductors with reference to international standard soft copper ("sgr"=5.8xc3x97107 [s/m])), and K represents a coefficient decided by the sectional structure of the microstrip line and the frequency.
As clearly understood from the above equations, the transmission loss, i.e., the attenuation constant xcex1, is increased in proportion to the relative permittivity xcex5r, the dielectric dissipation factor tanxcex4 and the frequency f Thus, a material having the minimum relative permittivity must be selected as the substrate material in the high frequency band of the millimeter waves, in order to reduce transmission loss. However, alumina having high relative permittivity of 9 to 10 increases transmission loss.
In consideration of handling in assembling, the strength of the microstrip substrate must be in excess of a constant value.
In order to solve these problems, Japanese Patent Laying-Open No. 8-228105, for example, discloses a technique of employing porous ceramics having open pores for a dielectric substrate. However, such porous ceramics having open pores also has the following problems:
(1) Airtightness and Dielectric Loss
(i) It is difficult to control water absorption due to open pores, and reduction of reliability specific to the ceramics sometimes results from storage of water vapor or the like. Particularly in a high-frequency band, a small amount of moisture-absorbing component or OH groups formed on the surface result in remarkable dielectric loss. This is because water exhibits a remarkably large dielectric dissipation factor tan8 of 0.1 to 1 in a frequency band of at least 1 GHz and dielectric loss is increased in this case also when the permittivity is reduced.
(ii) A high-frequency package member or the like essentially requires hermetic sealing. However, a porous material formed by open pores has no airtightness and cannot be applied to usage requiring airtightness due to apprehension for discharge of adsorbed gas in usage or the like.
(2) Surface Roughness
It is difficult to flatten a porous body formed by open pores also when working the surface, and hence radiation loss is caused from the irregular surface or conductor loss on a surface conductor is so increased that it is difficult to attain precise circuit formation.
(3) Via Hole
When a matrix formed with a through hole filled up with metal paste is a porous body formed by open pores, the metal paste infiltrates into the portion other than the through hole to reduce insulation resistance or increase conductor loss.
For example, Japanese Patent Laying-Open No. 4-88699 or Japanese Patent Laying-Open No. 4-125990 discloses a method also employing a dense body in order to partially solve such problems related to the porous body. According to this method, however, it is difficult to completely ensure airtightness although the dense body compensates for the strength of the porous body, and dielectric loss is unavoidably increased due to temporarily formed surface groups or moisture absorption. Further, the dense layer and the porous layer have different shrinkage factors, leading to stress or cracking when these layers are stacked or combined with each other.
For example, Japanese Patent Laying-Open No. 64-33946 or Japanese Patent Laying-Open No. 3-177376 discloses a method dispersing a porous material into resin or impregnating the porous body with resin. However, employment of the resin results in reduction of heat resistance and high airtightness cannot be attained due to employment of the resin, while dielectric loss is relatively increased as compared with single ceramics. Further, the porous material must be subjected to specific treatment such as surface treatment.
An object of the present invention is to provide porous ceramics having homogeneous and fine closed pores and a method of preparing the same.
Another object of the present invention is to provide surface-smoothed porous ceramics preparable by a method excellent in productivity and a method of preparing the same.
Still another object of the present invention is to provide a microstrip substrate excellent in airtightness and heat resistance by reducing high-frequency transmission loss.
Porous ceramics according to a first aspect of the present invention has relative density of less than 70% with a ratio of closed pores of at least 50% among all pores. Further, the relative density is less than 50%, and the ratio of closed pores among all pores is at least 90%. While pores are defined between grains 101a in general porous ceramics 101 as schematically shown in FIG. 2, grains la are hollowed in porous ceramics according to the present invention as schematically shown in FIG. 1, and hence a dense part (skeletal part) has a continuous network structure. Further, the porous ceramics containing no coarse holes 1a has superior mechanical strength to the conventional porous ceramics 101, and attains high heat conductivity depending on conditions. Particularly in an arbitrary section of the porous ceramics 1 having such a structure that holes 1a of a uniform diameter are dispersed due to hollowing of grains, the radii r1 and r2 of a pair of adjacent holes 1a and the width b of the ceramic part 1 can be set to (r1+r2)/b greater than 1. More preferably, (r1+r2)/b greater than 2. The structural phase of the porous ceramics consists of ceramics and an oxynitride phase. Further, the said ceramics contains at least any material selected from a group consisting of silicon nitride, silicon oxide, aluminum nitride and aluminum oxide.
The present invention is also directed to a ceramic circuit board having at least a partial insulating layer consisting of the said porous ceramics.
The porous ceramics according to the present invention can be obtained by a method of preparing a green compact consisting of metallic powder forming a precursor for the porous ceramics and heat-treating the green compact in reactive gas. Further, porous ceramics consisting of hollowed ceramic grains can be obtained by heat-treating the said green compact under microwave irradiation. The metallic powder is silicon, and the porous ceramics is silicon nitride or silicon oxide. Alternatively, the metallic powder is aluminum, and the porous ceramics is aluminum nitride or aluminum oxide.
Porous ceramics according to a second aspect of the present invention consists of ceramics having surface roughness (Ra) of less than 0.5 xcexcm and porosity of at least 30%.
Preferably in the aforementioned second aspect, the element forming the surface area of the ceramics includes the element forming the ceramics at a composition ratio different from the internal composition ratio of the ceramics.
Preferably in the aforementioned second aspect, the ceramics contains at least any material selected from a group consisting of alumina, silica, silicon nitride, aluminum nitride and silicon carbide.
Preferably in the aforementioned second aspect, the main phase of the ceramics is silicon nitride with aluminum (Al) contained in the surface area of the ceramics.
Preferably in the aforementioned second aspect, a thin film of a metal, an oxide or a nitride is formed on the surface.
Preferably in the aforementioned second aspect, a patterned metallic conductor is formed on the surface.
Preferably in the aforementioned second aspect, the surface of the porous ceramics is flattened by working the surface of the porous ceramics having porosity of at least 30% through solid phase reaction between abrasive grains and ceramics.
A microstrip substrate according to the present invention comprises a substrate, a microstrip line consisting of a conductor formed on the surface of the substrate and a base layer including at least a metallic plate or a metallized layer formed on the back surface of the substrate, and the substrate includes a ceramic porous body having porosity of at least 30% with a ratio of closed pores of at least 50% among all pores. The term porosity indicates the ratio of voids occupying the volume of the substrate.
In the microstrip substrate according to the present invention, the ceramic porous body is employed for the substrate. Thus, a substrate having heat resistance of at least 500xc2x0 C. can be proposed. Further, a substrate having relative permittivity smaller than the relative permittivity of conventional glass (SiO2) can be implemented by controlling the porosity of the ceramic porous body.
The porosity of the ceramic porous body is at least 30%. If the porosity is less than 30%, the relative permittivity exceeds the relative permittivity intrinsic to silica glass (SiO2) depending on the material for the ceramic porous material, and relative permittivity lower than that of a dielectric substrate consisting of conventional glass cannot be implemented.
The ratio of closed pores among all pores is at least 50%, whereby water absorption can be readily controlled and dielectric loss resulting from water absorption or the like can be reduced while hermetic sealing is simplified and hence the microstrip substrate can be applied to usage requiring airtightness. The closed pores are at the high ratio of at least 50%, and hence the surface can be readily flatly worked for suppressing radiation loss from surface irregularity or conductor loss in a surface conductor. Also when a through hole is formed, metal paste can be inhibited from infiltrating into a part other than the through hole and conductor loss resulting therefrom can be suppressed.
The ratio of the closed pores is at least 50%, whereby a microstrip substrate containing no component such as resin or organic matter causing loss or airtightness inhibition, reducing transmission loss of a high frequency and having excellent airtightness and heat resistance can be obtained.
Further, the base layer provided on the back surface of the substrate can remarkably reinforce the strength of the substrate. Also in a material such as a porous body having low strength, therefore, constant strength necessary for handling in assembling can be ensured by providing the base layer.
Preferably in the aforementioned microstrip substrate, the base layer has a glass substrate, a metallized layer formed on the surface of the glass substrate and a second metallized layer formed on the back surface of the glass substrate, and the metallized layer is arranged to be in contact with the back surface of the substrate.
Thus, the base layer can be prepared from a material of any structure in order to reinforce the strength of the porous body.
Preferably in the aforementioned microstrip substrate, the ceramic porous body has porosity of at least 50% with the ratio of closed pores of at least 80%, more preferably at least 90%, among all pores.
Thus, a microstrip substrate containing no component such as resin or organic matter causing loss or airtightness inhibition, reducing transmission loss of a high frequency and having excellent airtightness and heat resistance can be formed.
Preferably in the aforementioned microstrip substrate, the ceramic porous body is prepared from ceramics including at least any material selected from a group consisting of aluminum oxide, silicon nitride and aluminum nitride.
This material is selected in view of mechanical strength, the dielectric dissipation factor (tanxcex4) and heat resistance. The ceramics forming the substrate may be formed by compositing at least two materials selected from the above.
When alumina, silicon nitride or silicon oxide is employed as the material for the ceramic porous body, the relative permittivity can be reduced along the porosity in principle, as shown in Table 1.
Preferably in the aforementioned microstrip substrate, the radii r1 and r2 of a pair of adjacent holes and the width b of a ceramic part satisfy relation (r1+r2)/b greater than 1 in an arbitrary section of the ceramic porous body.
According to this structure, dielectric loss can be further reduced.